The long term goal of the proposed research is to understand the biology of the interactions of influenza viruses with host cell receptors at the molecular level. In past years, the applicant and coworkers have investigated the specificity of influenza isolates for their recognition of sialic acid containing carbohydrate receptor determinants on cell surface glycoproteins and glycolipids. Of particular interest was that human isolates with the H3 hemagglutinin specifically recognized sialosides with the SAa2, 6Gal linkage while avian and equine isolates with the same hemagglutinin gene bound the SAa2,3Gal linkage. Laboratory selection directed at differences in receptor specificity enabled isolation of variants with the opposite receptor type from both human and avian parent viruses. In collaboration with John Skehel and Don Wiley, the major differences in receptor specificity in the variants and the field isolates was shown to be due to a single amino acid at residue 226 which resides in the receptor binding pocket of the hemagglutinin. Insights have been gained into the basis of selection of these viruses in nature. The applicant's lab has also participated in the elucidation of the unique receptor determinant of influenza C virus as 9-0-acetyl-neuraminic acid. In the next project period the detailed molecular interactions between sialyloligosaccharide receptor determinants with and the hemagglutinin binding pocket will continue to be investigated with Don Wiley and John Skehel. Artificial sialic acid analogs will be screened to identify potent receptor site inhibitors. The role of the potent glycoprotein inhibitor, equine a2-macroglobulin, in maintaining the receptor specificity of equine H3 isolates will be studied. Differential virulence of the human H3 influenza viruses and receptor variants will be evaluated in ferrets and primary pig trachea epithelial cells. Artificial sialic acid receptor determinants of influenza C virus will be identified, and the role of the unique receptor specificity of influenza C in its host range will be examined. The proposed studies will contribute to a more complete molecular description of a virus-receptor interaction than currently available for any other animal virus. Further insights are expected to be gained in understanding host mechanisms of selection of receptor variants. The results will also have potential bearing on the development of live vaccines and small molecular weight inhibitors of influenza virus infection.